(USA TODAY) -- Law enforcement authorities investigating simultaneous, deadly bombings that ripped through crowds at the Boston Marathon Monday were pursuing potential witnesses across the region and reviewing streams of video gathered from nearby security cameras, as dozens of additional federal authorities were dispatched to Boston to assist in a broad terror inquiry.

Boston Police Commissioner Ed Davis said no suspects were in custody, contrary to various news reports indicating that at least one person had been detained.

"We are questioning many people, but there is no suspect in custody,'' Davis said.

A federal law enforcement official said investigators were examining three possible explosive devices in addition to the two that were detonated on the marathon route, including one recovered in the Newton area. The official, who had been briefed on the matter but was not authorized to comment publicly, said investigators were attempting to determine whether the material represented a threat.

The two devices detonated on the marathon route were described by the official as relatively small and possibly containing small ball bearings or BB gun pellets designed to serve as shrapnel.

It was unclear whether the devices were remotely detonated or included timers, the official said, adding that no conclusions had been drawn on whether an organized group or lone wolf had been responsible for the attack.

The official said authorities were questioning one person who was seen fleeing from the scene and sought treatment at a hospital. It was not known whether the person represented more than a witness to the incident.

The FBI in Boston said its agents are assisting Boston police. "The situation remains fluid, and it remains too early to establish the cause and motivation," the FBI said in a statement.

In a brief statement from the White House, President Obama also indicated that the source of the attacks was unknown: "We still don't know who did this or why.''

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Though Obama cautioned against jumping to conclusions about the motive for the attack, a senior administration official said it was "clearly an act of terror and will be approached as act of terror."

"We don't yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic," the senior administration official said. The official, who was not authorized to speak publicly, spoke on the condition of anonymity.

House Homeland Security Chairman Michael McCaul, R-Texas, spoke to reporters outside the House chamber late Monday.

"It has all the hallmarks of an act of terrorism," he said. "The White House is not calling it an act of terrorism yet. I am. We just don't know if it's foreign or domestic."

"One piece of evidence I'm particularly interested in is whether these devices contained ball bearings, because that's the signature of an IED," McCaul said. IEDs, or improvised explosive devices, are frequently used by al-Qaeda-linked groups and would suggest a foreign link, he said.

He noted that many similar planned attacks have been thwarted in recent years - the Times Square plot and the underwear bomber. "Unfortunately this one was successful."

Commissioner Davis said police had been dispatched to hospitals, hotels and elsewhere to identify possible witnesses to the dual blasts that exploded 50 to 100 yards apart, near the the iconic race's finish line.

The commissioner initially said police were treating a third incident at the John F. Kennedy Library as related, but later a police spokeswoman said the library incident was fire-related.

Investigators cordoned off the chaotic marathon crime scene, which was littered with debris and abandoned backpacks. Davis said the packs were being examined for other possible explosive devices.

"Anything that is unattended is being treated as a potential problem,'' Davis said. He said some were detonated in controlled blasts as a precaution.